My reply is only acquired knowledge but I'm pretty sure 2668 Griffin retro fitted cruise control. If not though, it's an available accessory and probably on Fleabay too. It's also suggested that the loom has the connector in there waiting for a switch..whether the computer needs telling it's there I'm not sure. Probably be a bit of difference between the manual and Dualogic box?
maybe ask 2668GRIFFIN
AFAIK,
the wiring / electronics is all there, you just need to fit a switch on the column to enable it..,
that's what happens with the ducato.. and they are all built by the same outfit,
guy in ducato section did it for @ £10 at maplin..,